Generally yes there are a few people that continue to spectate right up until they close down the track for the night.
Around 5:30-6:00 if you watch the entry road traffic you will notice a shift where most of the traffic is out bound rather than both ways or inbound as it is in the morning. People that eat off the salt at the casinos or who have just had enough sun for the day begin to filter out during that 5:30-6:30 time period.
It also depends a bit on the weather and the track situation. If we are on a wind hold, folks will leave a bit earlier, if there is an incident like a blown engine that closes down the track for clean up late in the day, many times folks will figure it is not worth sitting through a 30 minute shutdown to only see another 15-45 minutes of runs.
No hard and fast rules, but by watching the entry road traffic you can get a sense of when most of the folks are heading back to town for the day. The rat rodders that show up out on the salt will like a flock of birds heading for water in the evening seem to all start heading out about the same time. When you see a lot of the classic rods headed out, you know it is about time for them to head into Wendover for their car show gathering and a bite to eat.
Larry